Noise reduction circuit

ABSTRACT

A noise reduction circuit in which a video signal (S I ) is divided into a first signal (S L ) having low frequency component and a second signal (S H1 ) having high frequency component, the above second signal (S H1 ) is divided into a third signal (S H2 ) having an amplitude more than a predetermined value and a fourth signal (S H3 ) having an amplitute less than the predetermined value, a noise reduction process using a delay circuit (12) is applied to the above fourth signal (S H3 ) to obtain a fifth signal (S H4 ), and the above first signal (S L ), third signal (S H2 ) and fifth signal (S H4 ) are added, the noise reduction circuit being applicable to apply to a television receiver, etc.

DESCRIPTION

1. Technical Field

This invention relates to a noise reduction ciruit for video signals used in, for instance, a television receiver.

2. Background Art

As a noise reduction circuit used in a television receiver or the like, one shown in FIG. 6 is hitherto known.

In FIG. 6, input video signal S_(I) fed at an input terminal 1 and containing noise passes through a low-pass filter 2 to become signal S_(L), the low frequency component in which noises has been removed. Signal S_(L) is supplied to an adder 3. The above input signal S_(I) is also supplied to a high-pass filter 4 to obtain signal S_(H1), the high frequency component containing the noise component from high-pass filter 4. Signal S_(H1) is supplied to a limiter 5. Limiter 5 has the input/output characteristic as shown in FIG. 7, in which the output signal becomes zero when the level of input signal is within the range between V₁ and V_(Z). Accordingly, a signal having a level above the above range of V₁ to V₂, that is, signal S_(H2) in which noise component has been removed, is obtained from the above high frequency component signal S_(H1) at limiter 5. Signal S_(H2) is supplied to adder 3 to be added to signal S_(L) in which the above high frequency component has been removed. As a result, output signal S_(O) in which noise is reduced is obtained at an output terminal 6.

Further, a conventional noise reduction circuit using a delay circuit such as a field memory or a which frame memory is known, in which difference signal between the signal obtained by delaying input signal S_(I) through a delay circuit such as a field memory or a frame memory, and input signal S_(I) a is obtained, and the difference signal is subtracted form input signal S_(I). Since the above difference signal is considered a noise component when there is no movement within a field or a frame in input signal S_(I), noise can be reduced by that noise component being subtracted from input signal S_(I). Accordingly, this circuit is provided with a movement detection ciruit for detecting movement is input signal S_(I). When movement is detected, the above difference signal is considered noise component so inhibited from being subtracted from input signal S_(I).

Another noise reduction circuit is proposed in Patent Kokai No. Sho 55-127737 by the present applicant.

DISCLOSURE OF THE INVENTION

As described above, in the noise reduction ciruit in FIG. 6, all low level components (ones within the range of V₁ to V₂) in high frequency signal S_(H1) of input signal S_(I) are condsidered noises. In practice, however, it may be that a signal component is contained within the range of V₁ to V₂. In that case, if the signal component is removed by limiter 5, the definition of image is lowered.

The noise reduction circuit using a field memory or a frame memory as described above has the further defects that it requires a large capacity memory in order not to lower the definition of image, and the circuit configuration becomes larger because the movement detection circuit is required.

For overcoming the above defects, in a noise reduction circuit according to the invention, a video signal is divided into a first signal having the low frequency component and a second signal having the high frequency component, the second signal is divided into a third signal having an amplitude more than a predetermined value and a fourth signal having an amplitude less than the predetermined value, a fifth signal is obtained by applying a noise reduction process using a delay circuit to the above fourth signal and the above first, third and fifth signals are added.

In the noise reduction circuit according to the invention, the noise reduction process using a delay circuit is effected as to the signal having an amplitude less than the predetermined value in the signal of the high frequency component, i.e. the fourth signal, so that the signal component of high frequency which is removed together with the noise component in a conventional noise reduction circuit is compensated for.

Accordingly, in the noise reduction circuit according to the invention, the definition if image is never lowered, and a large capacity memory is not required for the delay circuit. Since the movement component is low frequency, it is not contained by the high frequency component so the movement detection circuit can be omitted.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a first embodiment of the invention;

FIG. 2 is a graph showing a characteristic of the limiter of FIG. 1;

FIG. 3 is a block diagram showing a second embodiment of the invention;

FIGS. 4A-4G wave form charts of signals at points of FIG. 3;

FIG. 5 is a circuit diagram showing an embodiment of an actual circuit configuration of FIG. 3;

FIG. 6 is a block diagram of a conventional noise reduction circuit; and

FIG. 7 is a graph showing the characteristic of the limiter of FIG. 6.

THE BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a first embodiment of the invention, in which parts corresponding to those of FIG. 6 are denoted by the same references as those of FIG. 6 to omit the descriptions.

In FIG. 1, input terminal 1, low-pass filter 2 for obtaining the first signal S_(L), adder 3, high-pass filter 4 for obtaining the second signal S_(H1) and limiter 5 for obtaining the third signal S_(H2) have the same constructions as those of FIG. 6. Output signal S_(L) +S_(H2) of the above adder 3 is supplied to adder 7.

The above high frequency signal S_(H1), as the second signal, as obtained from highpass filter 4, is supplied to limiter 8. Limiter 8 has the input/output characteristic shown in FIG. 2, in which output signal is obtained when input signal is within the range of V₁ to V₂, and output becomes constant when the level of input signal is out of the range of V₁ to V₂. Therefore, a signal between V₁ and V₂ in high frequency signal S_(H1), that is, signal S_(H3), the fourth signal containing a noise component is obtained from limiter 8. An noise reduction process using a delay circuit is effected as to signal S_(H3) as follows. Signal S_(H3) is supplied to adders 9 and 10, and to A/D converter 11 to be converted into digital signal, and then written in field memory 12 as in the above-mentioned delay circuit. The digital signal read from memory 12 is converted into analog signal S_(H31) at D/A converter 13. Therefore, this analogue signal S_(H31) is delayed from the above signal S.sub. H3 by a field.

Signal S_(H31) is supplied to adder 10 to be subtracted from the above signal S_(H3). Difference signal S_(H3) -S_(H31) between signal S_(H3) and signal S_(H31) obtained from adder 10 is to represent a noise componet for the reason described below. Signal S_(H3) -S_(H31) of this noise component is multiplied by a predetermined coefficient K at multiplier 14, and then subtracted from signal S_(H3) at adder 9. Accordingly, high frequency signal S_(H4), as the fifth signal in which noises are reduced, is obtained from adder 7. Signal S_(H4) is added to the above signal S_(L) +S_(H2) to be output signal S_(O) which is output at output terminal 6.

Next, it will be described that the above difference signal S_(H3) -S_(H31) a is noise component.

If input signal S_(I) is supplied directly to A/D converter 11 and adders 9 and 10, output signal of adder 10 represents the noise component or movement component of the image. For instance, in the case that input signal S_(I) is a still video signal, when a 1-field delayed signal obtained from the D/A converter 13 is subtracted from input signal at adder 10, the subtraction output should become zero even if there is noise. Therefore, if the subtraction output itself is not zero, the subtraction output is noise. In the case that the input signal is not a still video signal, the subtraction output is detected whether it is noise or a movement component. If it is a movement component, it is required that the coefficient K of the next multiplier 14 is set to zero so as not to subtract the movement component. For this purpose, a movement detection circuit is required.

In this embodiment, high frequency signal S_(H3) is supplied to A/D converter 11 and adders 9 and 10. Since any movement component of image is low frequency in general, little movement component is contained by the above signal S_(H3). Accordingly, if the difference signal S_(H3) -S_(H31) obtained from adder 10 is not zero, it may be substantially considered to be a noise component. Therefore, a movement detection circuit is not required in this embodiment.

If a movement detection circuit is provided for improving the accuracy, the movement detection circuit is disposed between the input and output of memory 12 or it is arranged so that the above difference signal S_(H3) -S_(H31) is added thereto. In that case, the movement detection circuit may generate a movement detection signal by judging that there is movement when the input level is beyond a predetermined level. The movement detection signal is supplied to multiplier 14 to control the coefficient K.

In this embodiment, since noise detection using memory 12 is effected as to signal S_(H3) having the narrow amplitude passed through limiter 8, the capacity of memory 12 can be decreased in comparison with the case in which the noise detection using memory 12 is effected as to the entire band of input signal S_(I). According to experiments, it has been affirmed that the number of bits of signal processed by A/D converter 11, memory 12 and D/A converter 13 can be decreased from a conventional 8 bits to 6 bits.

FIG. 3 shows a second embodiment, in which parts corresponding to those of FIG. 1 are denoted by the same references as those of FIG. 1 to omit the descriptions.

In this embodiment, signal S_(H31) is obtained by delaying by a field at memory 12 signal S_(H4), created when the above signal S_(H3) is passed through adder 9, and subtracted from signal S_(H3) at adder 9, is the difference signal S_(H3) -S_(H31), which is passed through multiplier 14 and subtracted from signal S_(H3). Namely, by feeding back, after delaying by a field, signal S_(H4) obtained from adder 9 in which noise is reduced, noise contained by signal S_(H4) is removed gradually.

FIG. 4 show wave forms of signals S_(I), S_(L), S_(H1), S_(H2), S_(H3), S_(H4) and S_(O) in FIG. 3, respectively. In FIGS. 3 and 4, signal S_(L) in which the noise component and high frequency of video part are removed is obtained by the manner that input signal S_(I) having noise N passes through low-pass filter 2. Signal S_(H1) containing the noise component and high frequency video part is obtained by signal S_(I) passing through high-pass filter 4. Signal S_(H2) having an amplitude more than V₁ and V₂ in signal S_(H1) is obtained by signal S_(H1) passing through limiter 5 having the characteristic shown in FIG. 7. Signal S_(H3) having the narrow amplitude between V₁ and V₂ is obtained by signal S_(H1) passing through limiter 8 having the characteristic shown in FIG. 2. Signal S_(H4) in which the noise component is reduced is obtained from adder 9. Output signal S_(O) is obtained by adding signal S.sub. H4 to signal S_(L) +S_(H2) at adder 7. Although some noise remains in this output signal S_(O), even if a signal component is contained by signal S_(H2) obtained from limiter 5, it can be compensated with signal S_(H4) to avoid lowering the definition of the image.

FIG. 5 shows a concrete circuit configuration of each of circuit blocks of FIG. 3, in which parts corresponding to those of FIG. 3 are denoted by the same references as those of FIG. 4.

In FIG. 5, the low-pass filter 2 comprises resistance R₁ and capacitor C₁, the output signal S_(L) of which is taken through an emitter follower of transistor Q₁. The high-pass filter 4 comprises capacitor C₂ and resisrances R₂ and R₃.

The limiter 5 is constructed by connecting in parallel a differential circuit of transistors Q₂ and Q₃ and a differential circuit of transistors Q₄ and Q₅. Output signal S_(H1) of high-pass filter 4 is applied to the bases of transistors Q₂ and Q₄, and constant voltages V₁ and V₂ are applied to the bases of transistors Q₃ and Q₅. Accordingly, transistors Q₂ is turned on when signal S_(H1) is beyond V₁, and transistor Q₄ is turned on when signal S_(H1) is beyond V₂. The common emitter outputs of the two differential circuits are added to each other through resistances R₄ and R₅ to be output as signal S_(H2) through an emitter follower of transistor Q₆.

The limiter 8 is constructed by connecting in series a differential circuit of transistors Q₇ and Q₈ and a differential circuit of transistors Q₉ and Q₁₀ through transistors Q₁₁. The voltage V₂ is applied to the base of transistor Q₈ and the above voltage V₁ is applied to the base of transistor Q₁₀. Accordingly, when signal S_(H1) is beyond V₂, transistor Q₇ is turned on so that its output signal is applied from its emitter to the base of transistor Q₉ through transistor Q₁₁. When this base voltage is beyond V₁, transistor Q₉ is turned on so that its output signal is amplified by amplifier 15 comprising transistors Q₁₂, Q₁₃ and Q₁₄ to obtain signal S_(H3). The above transistor Q₁₁ is for compensating for the voltage drop between the base and emitter of transistor Q₇.

The signal S_(H3) is supplied to A/D converter 11 after being multiplied by K at amplifier 14 (multiplier 14 in FIG. 3) comprising transistors Q₁₅ and Q₁₆, and added to the above signal S_(H3) through resistances R₆ and R₇. The added signal is taken as signal S_(H4) through an emitter follower of transistor Q₁₇.

A signal converted into, for instance, a 6-bit digital signal by A/D converter 11 is reconverted into an analog signal by D/A converter 13 after being delayed by 1-field memory 12, and then inverted by inversion amplifier 16 comprising transistors Q₁₈ and Q₁₉. This inverted signal is added to the signal S_(H3) through resistance R₉ and R₁₀, and the added signal is supplied to the amplifier 14.

Signals S_(L), S_(H2) and S_(H4) obtained as above are added through resistances R₁₁, R₁₂ and R₁₃ to obtain output signal S_(O) at output terminal 6.

Although field memory 12 is used in the first and second embodiments, a frame memory may be used of course.

According to the present invention, the definition of image is never lowered, and it has no need of a field memory or a frame memory of especially large capacity. Moreover, a noise reduction circuit in which a movement detection circuit is omitted can be provided. Therefore, the present invention is capable of being applied to a television receiver, etc. 

We claim:
 1. A method of reducing noise in a video signal comprising the steps of:dividing said video signal into a first signal containing the low frequency component of said video signal and a second signal containing the high frequency component of said video signal; dividing said second signal into a third signal having an amplitude more than a predetermined value and a fourth signal having an amplitude less than said predetermined value and containing noise; reducing noise in said fourth signal by means of a delay circuit to create a fifth signal; and adding said first, third and fifth signals together.
 2. A method of reducing noise in a video signal according to claim 1, in which said delay circuit is a field delay circuit or frame delay circuit.
 3. A method of reducing noise in a video signal according to claim 2, in which the step of reducing noise in said fourth signal by means of a delay circuit to create a fifth circuit further comprises the steps of:creating a difference signal by delaying said fourth signal and subtracting said delayed signal from said fourth signal; creating a fifth signal by subtracting said difference signal from said fourth signal.
 4. A metehod of reducing noise in a video signal according to claim 1 or 2 further comprising the step of providing an analog to digital converter at the input side of said delay circuit and a digital to analog converter at the output side thereof.
 5. A method of reducing noise in a video signal according to claim 4 in which said difference signal is determined by a feed-back loop.
 6. A noise reduction circuit in a which a video signal is divided into a first signal containing the low frequency component of said video signal and a second signal containing the high frequency component of said video signal, comprising:limiter means for dividing said second signal into a third signal having an amplitude more than a predetermined value and a fourth signal having an amplitude less than said predetermined value and containing noise; delay circuit means for reducing noise in said fourth signal to create a fifth signal; and adding means for adding said first, third and fifth signals.
 7. A noise reduction circuit according to claim 6, in which the delay circuit means comprises:a delay circuit for delaying said fourth signal; subtracting means for creating a difference signal by subtracting said delayed signal from said fourth signal; second subtracting means for creating a fifth signal by subtracting said difference signal from said fourth signal.
 8. A noise reduction circuit according to claim 7, in which said delay circuit is a field delay circuit or frame delay circuit.
 9. A noise reduction circuit according to claim 7 further comprising an analog to digital converter at the input side of said delay circuit and a digital to analog converter at the output side thereof.
 10. A noise reduction circuit according to claim 6, in which the delay circuit means comprises:a delay circuit for producing a delay signal; subtracting means for creating a difference signal by subtracting said delay signal from said fourth signal; second subtracting means for creating a fifth signal by subtracting said difference signal from said fourth signal; and feedback means for feeding said fifth signal back to said delay circuit.
 11. A noise reduction circuit according to claim 10 further comprising an analog to digital converter at the input side of said delay circuit and a digital to analog converter at the output side thereof.
 12. a noise reduction circuit according to claim 10, in which said delay circuit is a field delay circuit or frame delay circuit. 